1. Field of the Invention
The present invention relates to calibration of lithographic projection apparatus, a mask for use in calibration of lithographic apparatus, a lithographic projection apparatus, a method of manufacturing a device using a lithographic apparatus, and a device manufactured by the method.
2. Description of the Related Art
The term xe2x80x9cpatterning devicexe2x80x9d as here employed should be broadly interpreted as referring to a device that can be used to endow an incoming radiation beam with a patterned cross section, corresponding to a pattern that is to be created in a target portion of the substrate. The term xe2x80x9clight valvexe2x80x9d can also be used in this context. Generally, the pattern will correspond to a particular functional layer in a device being created in the target portion, such as an integrated circuit or other device (see below). An example of such a patterning device is a mask. The concept of a mask is well known in lithography, and it includes mask types such as binary, alternating phase-shift, and attenuated phase-shift, as well as various hybrid mask types. Placement of such a mask in the radiation beam causes selective transmission (in the case of a transmissive mask) or reflection (in the case of a reflective mask) of the radiation impinging on the mask, according to the pattern on the mask. In the case of a mask, the support structure will generally be a mask table, which ensures that the mask can be held at a desired position in the incoming radiation beam, and that it can be moved relative to the beam if so desired.
Another example of a patterning device is a programmable mirror array. One example of such an array is a matrix-addressable surface having a viscoelastic control layer and a reflective surface. The basic principle behind such an apparatus is that, for example, addressed areas of the reflective surface reflect incident light as diffracted light, whereas unaddressed areas reflect incident light as undiffracted light. Using an appropriate filter, the undiffracted light can be filtered out of the reflected beam, leaving only the diffracted light behind. In this manner, the beam becomes patterned according to the addressing pattern of the matrix-addressable surface. An alternative embodiment of a programmable mirror array employs a matrix arrangement of tiny mirrors, each of which can be individually tilted about an axis by applying a suitable localized electric field, or by employing piezoelectric actuators. Once again, the mirrors are matrix-addressable, such that addressed mirrors will reflect an incoming radiation beam in a different direction to unaddressed mirrors. In this manner, the reflected beam is patterned according to the addressing pattern of the matrix addressable mirrors. The required matrix addressing can be performed using suitable electronics. In both of the situations described above, the patterning device can comprise one or more programmable mirror arrays. More information on mirror arrays as here referred to can be seen, for example, from U.S. Pat. Nos. 5,296,891 and 5,523,193, and PCT patent applications WO 98/38597 and WO 98/33096, which are incorporated herein by reference. In the case of a programmable mirror array, the support structure may be embodied as a frame or table, for example, which may be fixed or movable as required.
A still further example of a patterning device is a programmable LCD array. An example of such a construction is given in U.S. Pat. No. 5,229,872, which is incorporated herein by reference. As above, the support structure in this case may be embodied as a frame or table, for example, which may be fixed or movable as required.
For purposes of simplicity, the rest of this text may, at certain locations, specifically direct itself to examples involving a mask and mask table. However, the general principles discussed in such instances should be seen in the broader context of the patterning device set forth above.
Lithographic projection apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such manufacturing, the patterning device may generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g. comprising one or more dies) on a substrate (silicon wafer) that has been coated with a layer of radiation-sensitive material (resist). In general, a single wafer will contain a whole network of adjacent target portions that are successively irradiated via the projection system, one at a time. In current apparatus, employing patterning by a mask on a mask table, a distinction can be made between two different types of machine. In one type of lithographic projection apparatus, each target portion is irradiated by exposing the entire mask pattern onto the target portion at once. Such an apparatus is commonly referred to as a wafer stepper. In an alternative apparatus, commonly referred to as a step and scan apparatus, each target portion is irradiated by progressively scanning the mask pattern under the projection beam in a given reference direction (the xe2x80x9cscanningxe2x80x9d direction) while synchronously scanning the substrate table parallel or anti-parallel to this direction. Since, in general, the projection system will have a magnification factor M (generally  less than 1), the speed V at which the substrate table is scanned will be a factor M times that at which the mask table is scanned. More information with regard to lithographic devices as here described can be seen, for example, from U.S. Pat. No. 6,046,792, incorporated herein by reference.
In a manufacturing process using a lithographic projection apparatus, a pattern (e.g. in a mask) is imaged onto a substrate that is at least partially covered by a layer of radiation-sensitive material (resist). Prior to this imaging, the substrate may undergo various procedures, such as priming, resist coating and a soft bake. After exposure, the substrate may be subjected to other procedures, such as a post exposure bake (PEB), development, a hard bake and measurement/inspection of the imaged features. This array of procedures is used as a basis to pattern an individual layer of a device, e.g. an IC. Such a patterned layer may then undergo various processes such as etching, ion-implantation (doping), metallization, oxidation, chemo-mechanical polishing, etc., all intended to finish off an individual layer. If several layers are required, then the whole procedure, or a variant thereof, will have to be repeated for each new layer. Eventually, an array of devices will be present on the substrate (wafer). These devices are then separated from one another by a technique such as dicing or sawing, whence the individual devices can be mounted on a carrier, connected to pins, etc. Further information regarding such processes can be obtained, for example, from the book xe2x80x9cMicrochip Fabrication: A Practical Guide to Semiconductor Processingxe2x80x9d, Third Edition, by Peter van Zant, McGraw Hill Publishing Co., 1997, ISBN 0-07-067250-4, incorporated herein by reference.
For the sake of simplicity, the projection system may hereinafter be referred to as the xe2x80x9clens.xe2x80x9d However, this term should be broadly interpreted as encompassing various types of projection systems, including refractive optics, reflective optics, and catadioptric systems, for example. The radiation system may also include components operating according to any of these design types for directing, shaping or controlling the projection beam of radiation, and such components may also be referred to below, collectively or singularly, as a xe2x80x9clensxe2x80x9d. Further, the lithographic apparatus may be of a type having two or more substrate tables (and/or two or more mask tables). In such xe2x80x9cmultiple stagexe2x80x9d devices the additional tables may be used in parallel, or preparatory procedures may be carried out on one or more tables while one or more other tables are being used for exposures. Dual stage lithographic apparatus are described, for example, in U.S. Pat. No. 5,969,441 and U.S. Pat. No. 6,262,796, incorporated herein by reference.
As the critical dimension (CD), i.e. the dimension of a feature or features, such as the gate width of a transistor, in which variations will cause undesirable variation in physical properties of the feature, in lithography shrinks, consistency of focus and exposure dose, both across a substrate and between substrates, becomes increasingly important. Traditionally, steppers used energy sensors to monitor the exposure dose by monitoring the output of the illumination system, and image sensors to probe the aerial image. Optimal settings were determined by xe2x80x9csend-ahead wafersxe2x80x9d i.e. wafers that are exposed, developed and measured in advance of a production run. In the send-ahead wafers, test structures were exposed in a focus energy matrix (FEM) and the best focus and energy settings were determined from examination of those test structures.
The use of an alignment system to monitor focus has been proposed and an extension of this technique to also measure exposure dose was disclosed in the article xe2x80x9cFocus and Exposure Dose Determination using Stepper Alignmentxe2x80x9d by Peter Dirksen et al, SPIE Vol. 2726/799 (1996). This article describes alignment markers that are modified to form a focus mark and a dose mark. One example of a known alignment mark comprises an array of four phase gratings, two of which have their grating lines aligned in a first direction and the other two having their grating lines perpendicular to the first direction. In the focus mark, the line of the grating is replaced by a chopped structure with a sub-resolution chop linewidth. For example, if the overall grating period is 16 xcexcm, the chop linewidth may be in the range of 0.7 to 0.25 xcexcm. In the dose mark, the grating line comprises a transparent part and a part with a transmission of about 0.4.
In the method described by Dirksen, the focus energy matrix is imaged on a substrate with a radiation sensitive layer, but the radiation sensitive layer is not processed. The resulting image is called a xe2x80x9clatent imagexe2x80x9d and is formed by thickness variations as a function of irradiation variations, resulting in phase delay of light diffracted at the mark image. By measuring the position of these modified marks with respect to regular alignment marks, an alignment offset (AO) shows up which is representative of focus and dose errors.
One disadvantage of this method is that the dose calibration must be done in the plane of best focus, because the dose AO is focus sensitive. Therefore, simultaneous calibration of focus and dose is difficult, if not impossible. Another disadvantage is that AO measurements of latent images are not as representative as AO measurements of processed images. For example, the phase delay properties of a latent image depend upon the resist properties and resist thickness, whereas further downstream in the process (i.e. after developing and stripping of the resist) such dependence is absent. Also, latent images producing easily measurable AOs can only be formed in resist layers that are thick with respect to typical processed resist layers, and are overexposed. Consequently, the doses used for the calibration process are not comparable to doses used in actual process steps and so the resulting measurements are not as representative.
The article xe2x80x9cMeasuring Photolithographic Overlay Accuracy and Critical Dimensions by Correlating Binarized Laplacian of Gaussian Convolutionsxe2x80x9d by H Keith Nishihara et al, IEEE Transactions on Pattern Analysis and Machine Intelligence vol. 10 no. 1 January 1988, discloses a method of measuring overlay accuracy and variations in critical dimensions using targets of random patterns of small elements at the minimum feature size of the photolithographic process. The markers are printed and observed with a camera. The camera image is then processed to determine alignment. By adding a grid, changes in the size of the imaged elements cause changes in alignment.
It is an object of the present invention to provide a calibration method that can provide a simultaneous focus and dose calibration more representative of actual processed conditions, whilst using existing alignment tools.
This and other objects are achieved according to the invention in a method of calibrating a lithographic projection apparatus, the apparatus including a radiation system constructed and arranged to supply a projection beam of radiation; a support structure constructed and arranged to support a patterning device, the patterning device constructed and arranged to pattern the projection beam according to a desired pattern; a substrate table to hold a substrate; and a projection system constructed and arranged to project the patterned beam onto a target portion of the substrate, the method including imaging at least one alignment marker provided in the patterning device onto a radiation sensitive layer on a substrate held by said substrate table, the at least one alignment marker including at least one grating and having a focus sensitive part and a dose sensitive part, the focus sensitive part and the dose sensitive part each having a plurality of dots on a contrasting background, the pluralities of dots being arranged such that variations in focus settings and dose settings in the imaging cause shifts in the apparent position of the at least one grating.
The invention also provides a patterning device constructed and arranged to pattern the projection beam in a lithographic projection apparatus according to a desired pattern, the patterning device being useable to project a pattern comprising at least one alignment marker including at least one grating having a focus sensitive part and a dose sensitive part, the focus sensitive part and the dose sensitive part each having a plurality of dots on a contrasting background, the pluralities of dots being arranged such that variations in focus settings and dose settings when projecting an image of the at least one alignment marker cause shifts in the apparent position of the at least one grating.
Preferably, the at least one alignment marker imaged on the radiation sensitive layer on the substrate is then developed and its position measured to provide alignment offsets which are representative of focus errors and dose delivered at substrate level. The position of the at least one alignment marker can be measured with a known alignment tool, providing reliable and consistent results. A complete calibration of the focus and dose settings of the apparatus can be obtained by imaging and developing a FEM whereby a plurality of images of the at least one alignment marker are imaged at different dose settings and focus settings in an array. The invention further provides a second stage to the calibration process whereby dose settings and focus settings of the apparatus are selected, with the focus setting displaced from a best focus position, and the at least one alignment marker is then imaged at a plurality of different positions on a substrate so as to provide full X-Y calibration of focus and dose for the apparatus, where the X and Y directions are substantially parallel to the plane of the substrate.
With the invention, focus and dose can be calibrated using a plurality of exposures of the at least one alignment marker on a single wafer and at focus and dose settings more applicable to those used in actual processes.
According to a further aspect of the invention there is provided a method of manufacturing a device using a lithographic apparatus, the method including providing a substrate that is at least partially covered by a layer of radiation-sensitive material; providing a projection beam of radiation using a radiation system; patterning the projection beam with a pattern in its cross-section; and projecting the patterned beam of radiation onto a target area of the layer of radiation-sensitive material, wherein the apparatus is first subjected to a focus and dose calibration as described above, and the results of that calibration are subsequently used in determining focus settings and dose settings employed in the projecting.
Although specific reference may be made in this text to the use of the apparatus according to the invention in the manufacture of ICs, it should be explicitly understood that such an apparatus has many other possible applications. For example, it may be employed in the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, liquid-crystal display panels, thin-film magnetic heads, etc. One of ordinary skill will appreciate that, in the context of such alternative applications, any use of the terms xe2x80x9creticlexe2x80x9d, xe2x80x9cwaferxe2x80x9d or xe2x80x9cdiexe2x80x9d in this text should be considered as being replaced by the more general terms xe2x80x9cmaskxe2x80x9d, xe2x80x9csubstratexe2x80x9d and xe2x80x9ctarget portionxe2x80x9d, respectively.
In the present document, the terms xe2x80x9cradiationxe2x80x9d and xe2x80x9cbeamxe2x80x9d are used to encompass all types of electromagnetic radiation, including ultraviolet radiation (e.g. with a wavelength of 365, 248, 193, 157 or 126 nm) and EUV (extreme ultra-violet radiation, e.g. having a wavelength in the range 5-20 nm).